1. Field of the Invention
The present invention relates to an image forming apparatus having an image bearing member and a development device to utilize magnetic toner for development from a developer bearing member to the image bearing member.
2. Description of the Related Art
In the related art, an image forming apparatus of an electrophotographic system has been known. In such an image forming apparatus, an exposure device irradiates light to an image bearing member to write an electrostatic image on the surface of the image bearing member after a primary charging device evenly charges the surface of the image bearing member. Then, a magnetic toner is utilized for development from a developer bearing member to the image bearing member. The magnetic toner utilized for development is transferred to a sheet by a transfer device, and then, the sheet is separated from the image bearing member. The magnetic toner is fixed on the sheet by applying heat and pressure by the fixing device to the sheet having the magnetic toner transferred.
When magnetic toner is utilized for development from the abovementioned developer bearing member to the image bearing member, the following phenomenon has been an issue. That is, there may be a case that a magnetic toner having opposite-polarity appears on a non-image portion of the electrostatic image with development while an image portion of the electrostatic image is developed with a normal magnetic toner. A possibility of fog occurrence due to a magnetic toner having opposite-polarity has been an issue. Japanese Patent Application Laid-Open No. 2004-333709 discusses an invention to suppress fog caused by a magnetic toner having opposite-polarity.
According to an image forming apparatus of the invention discussed in Japanese Patent Application Laid-Open No. 2004-333709, it is possible to discharge a magnetic toner having opposite-polarity by changing direct-current voltage applied to the developer bearing member from that during normal image forming. With this configuration, roughening, density decrease and fog due to selective development are suppressed and an excellent image can be formed while keeping a lifetime of the image bearing member without changing toner composition and body structure.
However, there may be a possibility that a magnetic toner is excessively consumed as a whole as a magnetic toner having opposite-polarity is frequently and excessively discharged. Further proceeding, there may be a possibility that the image forming apparatus is broken due to splashing of the magnetic toner at the vicinity of a developer container.
The present invention provides an image forming apparatus capable of suppressing excessive consumption of a magnetic toner for reducing fog occurrence on an output image.